1. Field of the Invention
The present invention generally relates to optical disks and optical disk apparatuses, and more particularly to an optical disk and an optical disk apparatus in which the frequency of erasing performed immediately before writing is reduced.
2. Description of the Prior Art
FIG. 1 is a diagram showing an overview of a conventional optical disk apparatus. Data is written on an optical disk 1 as follows. A bias magnetic field H is applied to the optical disk 1. In this case, a spot of a laser beam emitted from an optical head 2 is continuously projected onto a block on the optical disk 1. Thereby, the block is magnetized in the direction of the bias magnetic field H, so that data recorded on the block is erased. In order to write data on the above block, the bias magnetic field H is applied to the block in the reverse direction. When the optical disk 1 is revolted once, the laser beam spot is projected onto the block again. Thereby, the block is magnetized in the direction the reverse of the direction in the erased state, so that data can be written on the block. In the above-mentioned manner, the erasing operation must be performed before writing of data is performed.
FIG. 2 is a block diagram showing the format of the optical disk 1 prescribed in the ISO 10090 standard, the disclosure of which is hereby incorporated by reference. The optical disk 1 has a user accessible area 10 and a user inaccessible area 20. A user is free to write data or programs necessary to handle the optical disk 1 on the optical disk 1. The user inaccessible area 20 allows only the manufacturer of the optical disk 1 to write data necessary for disk management. That is, the user is not allowed to write data on the user inaccessible area 20.
More particularly, the user accessible area 10 accessible by the user in a normal way (mode) is defined between the third track and the 9996th track. The user accessible area 10 shown in FIG. 2 includes a RAM (rewritable) area 10a and a ROM area (read only area) 10b. The RAM area 10a is closer to the inner portion of the optical disk 1 than the ROM area 10b. The user can write data on the RAM area 10a and read data therefrom. The user is only allowed to read data from the ROM area 10b. The optical disk having the above user accessible area 10 is called a partial ROM type optical disk. It is also possible to design the entire user accessible area 10 so that it is comprised of only the RAM area 10a or the ROM area 10b.
Two defect management areas (DMA) 20m are provided so that the user accessible area 10 is sandwiched between them. Information necessary for disk management, such as a disk production number and date, is recorded on the defect management areas 20m. An inner control track area 20c is provided adjacent to the defect management area 20m located in the inner portion of the optical disk 1. An outer control track area 20c is provided adjacent to the disk management area 20m located in the outer portion of the optical disk 1 is provided. Data concerning the allocation of the RAM area 10a and the ROM area 10b is recorded on the inner and outer control track areas 20c, respectively.
In the conventional data writing process, the erasing operation is performed first, and the data writing operation on is performed second after the optical disk 1 is rotated once. Hence, an overwrite operation cannot be performed, and it thus takes a long time to write data on the optical disk 1.
In order to eliminate the above problem, the following improvements have been proposed. The first proposed improvement is to use two laser beam spots, one of the two spots being used for erasing and the other being used for writing. The second proposed improvement is to employ a magnetic modulation method using a magnetic floating head. The third proposed improvement is to use an optical disk having a multi-layer structure.
However, the first proposed improvement needs a complex optical system, and hence the optical disk apparatus is expensive. The second proposed improvement needs a process for floating the magnetic head on the side of the optical disk 1 opposite to the optical head 2. The above process cannot be interchanged with that of the conventional art. Further, head crash may occur as in the case of magnetic disks. Furthermore, it is necessary to provide a mechanism for moving the magnetic floating head. The third proposed improvement cannot provide any multi-layer structure which can be put to practical use.